Energy storage devices such as batteries with high energy density and power density, long cycle life and calendar life, good safety and low cost are in high demand to supply power for consumer electronic devices, electric vehicles (EVs) and smart grid energy storage. To date, lithium (Li)-ion batteries have been one of the most widely used energy storage systems for portable electronics and EVs. A typical commercial Li-ion battery consists of multiple cell stacks, each composed of an anode current collector/anode/separator/cathode/cathode current collector, soaked with a liquid electrolyte (typically 1 M LiPF6 in a carbonate solvent mixture). Common configurations for Li-ion batteries used in commercial electronics include: Cu/graphite/separator/LiCoO2/Al and Cu/graphite/separator/LiFePO4/Al. During the charge process, Li+ ions extracted from the cathode materials, diffuse through the electrolyte-soaked separator, and then intercalate into the anode material (i.e., graphite). The process is reversed during the discharge process. In these batteries, the usable energy is determined by the identity and amount of active materials present, but the total weight and to a large extent cost are determined by all of the materials, with sizable contributions from the inactive components such as the separator, current collectors, and packaging materials.
Li metal has an extremely high theoretical specific capacity (3860 mAh g−1), low density (0.59 g cm−3) and the lowest negative electrochemical potential (−3.040 V vs. standard hydrogen electrode); thus rechargeable Li metal batteries have been investigated extensively during the last 40 years (M. S. Whittingham, Proceedings of the IEEE 2012, 100, 1518-1534; D. Aurbach and Y. Cohen, Journal of The Electrochemical Society, 1996, 143, 3525-3532). Li metal is also the basis for Li-air batteries and Li-sulfur batteries. Unfortunately, rechargeable batteries based on Li metal anode have not yet been commercialized in large scale. There are two main barriers to the development of rechargeable Li metal batteries: one is the growth of Li dendrites during repeated charge/discharge processes, and another is the low Coulombic efficiency (CE) of these processes. These two barriers consequently lead to two critical problems for the Li anode: one is safety hazards because of potential internal short circuits and the high surface area of the active material resulting in high reactivity; another is the short cycle life of such batteries due to low CE of Li cycling. Although low CE can be partially compensated by the inclusion of an excess amount of Li metal, for example, a 300% excess of Li was a common feature in the early development of Li metal batteries—but the dendrite-growth related battery failure (sometimes dramatic failure that leads to fire and/or other hazards), and the emergence of Li-ion batteries have largely diminished industry's efforts devoted to the development of rechargeable Li metal batteries since the early 1990s.